The present invention relates to a digital scanner installed in a facsimile apparatus, an electronic copier or the like for reading a document and, more particularly, to a technique for quantizing a video signal produced by scanning a document.
A digital scanner which has heretofore been built in an image reader of a digital copier and others includes a charge coupled device (CCD) for scanning a document and converting the resulting optical signal to an analog electrical signal, an amplifier for amplifying the analog signal to a certain level which is easy to process, and a peak hold circuit and an analog-to-digital (AD) converter which cooperate to convert the amplified analog signal to a digital signal using a predetermined threshold level. The problem with such a digital scanner is that since a predetermined threshold voltage applied to the AD converter is fixed, the number of bits for quantizing the signal is determined solely by the AD converter itself, that is, by the fixed predetermined threshold voltage.
Meanwhile, a two-level scanner heretofore built in a digital facsimile apparatus or the like also includes a CCD and an amplifier and, in addition, a comparator which receives an analog signal output from the amplifier at one of its input terminals. In such a two-level scanner, a fixed threshold voltage of a predetermined level is applied to the other input of the comparator and, hence, only a two-level signal processed with respect to the predetermined level can be produced from the comparator.
Any of the prior art scanners, therefore, is incapable of reading images carried on a document in multiple tones. An implementation for settling this situation has been keenly demanded in the imaging art.